Power semiconductor modules have one or a plurality of semiconductor chips which have to be connected up to connections of the module and/or to one another.
For this purpose, the semiconductor-chips are usually mounted on a carrier by one side and, on the other side, are electrically connected by means of bonding wires to other semiconductor chips or to external connections of the module.
The advancing miniaturization of the semiconductor chips means that the current density in the semiconductor chips rises with the chip area remaining the same, which makes it more difficult to achieve sufficient dissipation of heat from the semiconductor chips. In particular that side of a semiconductor chip which is contact-connected with bonding wires makes no significant contribution to the heat dissipation from the chip.
Furthermore, such bonding wires act as an inductance which together with the junction capacitance of a semiconductor chip formed as a power switch form a resonant circuit. The resonant frequency of such a resonator circuit is in the region of about 100 MHz in the case of a typical power switch having an area of 1 cm2.
If the load current is subjected to chopping in the power switch, then undesirable oscillations may thereby be excited owing to its great variation with respect to time.
Such power switches, in a power semiconductor module, are usually mounted on a DCB substrate (DCB=Direct Copper Bonding) comprising a copper-coated aluminum oxide ceramic. The copper coating of such a DCB substrate together with the conduction resistance of the bonding wires used for the connection of the power switch form a parasitic low-pass filter which, however, manifests a blocking effect sufficient for suppressing the abovementioned resonant frequency only at frequencies from approximately 1 THz.
A further problem occurs in power semiconductor modules comprising a plurality of drivable semiconductor chips, for example if the load paths thereof are connected in parallel and the semiconductor chips are intended to be driven synchronously externally. This necessitates, for each of the drivable semiconductor chips, a series resistor which is connected upstream of the control input of the respective semiconductor chip and is generally integrated in the relevant semiconductor chip.
Together with the input capacitances of the semiconductor chips said series resistors form low-pass filters, whereby transfer oscillations of the load current flowing through the semiconductor chips are suppressed.
With this type of circuitry, however, the control connections of the semiconductor chips are no longer directly connected to a driving electronic unit arranged outside the semiconductor chips, which results in delays during the driving of the semiconductor chips.
This in turn requires an increased circuit complexity in the driving electronic unit, particularly if high switching speeds are required.
It can thus happen, primarily when turning off the semiconductor chips, on account of parasitic capacitances, that the maximum voltage permissible across the load paths of the semiconductor chips is exceeded.
Situations of this type can be avoided by the relevant semiconductor chips being momentarily switched on again, which as a result leads to a softer turn-off behavior of the semiconductor chips. However, the process of momentary switching-on again has to be effected very rapidly, for which reason the semiconductor chips have to be able to be driven correspondingly rapidly. However, the maximum switching frequency is limited by the resistors integrated in the semiconductor chips and by the input capacitances of the semiconductor chips.